As infrastructure has evolved and matured over the last decade, the way in which we build and deploy that infrastructure has — for the most part — kept pace. As the velocity of deployments increased, and practices such as continuous deployment and delivery became the norm, it became critical that we manage infrastructure and deploy applications in a similar way.
From this need arose "Infrastructure-as-Code" (IaC), the management of infrastructure (and everything therein, from networks to VMs to load balancers) that provides the foundation for your apps. By deploying and codifying your deployments in the same way, you establish one framework — one source of truth — for the state of configuration for everything from your infrastructure to your applications. As Jesse Robins and John Allspaw put it in Web Operations, IaC lets you: "Enable the reconstruction of the business from nothing but a source code repository, an application data backup, and bare metal resources."